The invention relates to an airbag consisting of at least one contact fabric and/or at least one filter fabric, the contact fabric having an air permeability &lt;10 l/dm.sup.2 .multidot.min and the filter fabric having an air permeability of 20-100 l/dm.sup.2 .multidot.min.
Airbags are usually manufactured from high strength woven fabrics. With these fabrics a specific air permeability is a particularly important requirement. As the airbag inflates, it is necessary that the fabric have a low air permeability to ensure rapid inflation, but upon the vehicle occupant impacting on the airbag some of the gas which has flowed into the airbag has to be able to escape in order that soft cushioning of the vehicle occupant may be ensured. Examples of various low air permeability fabric designs are as described in DE 4,009,611, EP-A 416 483 and EP-A 436 950.
This is why an airbag frequently consists of two parts having different air permeabilities. The part of the airbag against which the incoming gas flows upon inflation and upon which the vehicle occupant impacts as a result of an accident is usually known as the contact part. The part which, upon the vehicle occupant impacting on the airbag, permits a partial escape of the gas in the airbag is known as the filter part. This is because the fabric of the filter part additionally has to ensure that hot particles which have streamed into the airbag together with the generator-produced gas are intercepted, i.e. filtered off. Examples of airbags consisting of two parts are as shown in EP-A 454 213 and EP-A 442 373.
Some airbags are manufactured from contact fabrics only. In such cases openings are introduced into the airbag to allow the gas to escape. In the case of front passenger airbags sometimes only filter fabrics will be employed.
In order that two-part airbags may meet the requirements of rapid inflatability and gas escape facility, the two parts of the airbag have to have different air permeabilities. In the case of the contact part the air permeability should be below 10 l/dm.sup.2 .times.min at a test differential pressure of 500 Pa. The filter part is required to have an air permeability between 20 and 100 l/dm.sup.2 .times.min at a test differential pressure of 500 Pa, depending on vehicle type. In the airbag fabrics manufactured to date the low air permeability required for the contact part is frequently achieved by coating the fabrics. In addition to the higher production costs, however, coated fabrics have further disadvantages, such as the greater space required to accommodate the airbag in the steering wheel of a vehicle. Recently there have been developed processes which make it possible to manufacture fabrics of low air permeability without an additional coating.
In addition, a fabric for the manufacture of airbags has to meet a number of other requirements. For instance, the fabric has to have a minimum strength level for optimum functioning. Similarly, the fabric should be readily foldable in order that the space required to accommodate the airbag in the steering wheel may be minimized. This also explains the requirement of maximum lightness.
The manufacture of the airbag fabrics also should be inexpensive. Since, owing to the low air permeability required, comparatively closely woven fabrics have to be produced, it is necessary to weave with a high number of warp and filling threads per cm, i.e. with a high closeness value. The number of filling threads has a direct bearing on the production efficiency of the weaving step.